This invention relates to a process for preparing highly purified monomers of hydroxymethylbenzoic acid from toluic acid methyl ester compounds of a purity suitable for preparing linear polyester polymers.
Methyl hydroxymethylbenzoates have been suggested as monomers for poly(p-methylenebenzoate) polymers and poly(m-methylenebenzoate) polymers but must be of a purity better than 99.5% to be considered as monomer grade. Ordinary methods of purification of methyl hydroxymethylbenzoates such as crystallization and distillation are not suitable economically. If methyl p-hydroxymethylbenzoate is crystallized and recrystallized to obtain a pure monomer, the crystallization is performed with great difficulty and uneconomic yield losses occur because of difficulty in crystallizing and similar solubilities in the crystallizing media of unwanted impurities such as carboxybenzaldehydes. Distillation of methyl hydroxymethylbenzoate, either para or meta isomer, results in a less pure product than the starting material because of the formation of carboxybenzaldehydes among other by-products of the distillation.
p-Hydroxymethylbenzoic acid is believed to have been first prepared as early as 1872 by free-radical bromination of p-toluic acid to p-bromomethylbenzoic acid, hydrolysis with aqueous barium hydroxide and subsequent purification by recrystallization from water. Other methods for the preparation of p-hydroxymethylbenzoic acid and/or methyl p-hydroxymethylbenzoate have since been discovered, including:
(a) Hydrolysis of p-toluic acid and derivatives functionalized at the benzylic position, such as p-halomethylbenzoic acid and esters. PA1 (b) Hydrolysis of p-halomethylbenzonitriles, p-hydroxymethylbenzonitrile and p-chloro-toluyl chloride. PA1 (c) Oxidation of p-xylene and substituted p-xylenes, such as p-hydroxymethyltoluene, p-acetoxymethyltoluene and p-xylenediol, and oxidation of p-toluic acid, p-tolualdehyde, and derivatives. PA1 (d) Chloromethylation of benzoic acid and toluene derivatives. PA1 (e) Carboxylation of p-halolotoluene compounds via lithium salts. PA1 (f) Disproportionation of terephthaldehyde (Cannizzaro reaction). PA1 (g) Polarographic reduction of dimethyl terephthalate. PA1 (a) 4-50 g of acetoxylated methyl p-hydroxymethylbenzoate in 100 ml hexane heated to 20.degree.-50.degree. C. and then cooled to -77.degree. to 10.degree. C. PA1 (b) 20-60 g of acetoxylated methyl p-hydroxymethylbenzoate in 100 ml diethyl ether/hexane in a 1:1 ratio heated to 20.degree.-50.degree. C. and then cooled to -77.degree. to 10.degree. C. PA1 (c) other light hydrocarbon fractions such as petroleum ether, etc. can be used with equivalent quantities and at equivalent temperatures.
A successful synthesis of polymer-grade p-hydroxymethylbenzoic acid requires that the synthesis be highly selective, as p-hydroxymethylbenzoic acid is extremely difficult to purify especially from 4-carboxybenzaldehyde and terephthalic acid. High purity (99.7+%) p-hydroxymethylbenzoic acid has been obtained by recrystallization of crude products which did not originally contain any 4-carboxybenzaldehyde. However purification of p-hydroxymethylbenzoic acid containing p-toluic acid, terephthalic acid and 4-carboxybenzaldehyde has not been successful as no economically satisfactory method has been found for removal of 4-carboxybenzaldehyde. Consequently, a successful selective synthesis of p-hydroxymethylbenzoic acid must give a product free of 4-carboxybenzaldehyde. Other reported syntheses such as hydrolyses of p-halomethylbenzoic acid and derivatives and oxidations of p-toluic acid are not selective enough. These reactions give products that contain 4-carboxybenzaldehyde and terephthalic acid, which cannot be reduced to levels below 1-2% by usual purification methods. Terephthalic acid can be removed from p-hydroxymethylbenzoic acid after several recrystallizations from water but 4-carboxybenzaldehyde cannot.
The extent of the problem was recognized at least as far back as 1958 when Ludwig, Ramm and Wiegand, J. prakt. Chem. (4) 6, 103 (1958) stated, "presently known syntheses of p-hydroxymethylbenzoic acid offer no possibilities of producing this hydroxycarboxylic acid commercially, so that production of a polyester fiber from p-hydroxymethylbenzoic acid on a large scale in the forseeable future is unlikely." This conclusion was reached upon the basis of low molecular weight polymers of p-hydroxymethylbenzoic acid with viscosities of 0.2-0.4, a glass transition temperature (Tg) of 90.degree. C. and melting points of from 205 to 250.degree. C. The polymers were compared to poly(ethyleneterephthalate) of viscosity 0.6-1.0 with a Tg of 73.degree. C. and a melting point of 265.degree. C.
Procedures also exist for the preparation of m-hydroxymethylbenzoic acid. These syntheses are analogous to those described for preparation of p-hydroxymethylbenzoic acid and methyl p-hydroxymethylbenzoate and which have been shown to be unsuitable for preparation of high-purity monomer. These methods give products, i.e., m-hydroxymethylbenzoic acid and methyl m-hydroxymethylbenzoate, contaminated with toluic acid, carboxybenzaldehyde and dicarboxylic acid impurities that are difficult to remove.
We have now found a process for the preparation of highly purified monomers for homopolymers of p-hydroxymethylbenzoic acid and m-hydroxymethylbenzoic acid. The process involves the electrochemical side-chain acetoxylation of toluic acid methyl ester compounds to form the acetates of m and p-hydroxymethylbenzoic acid. The resulting monomers are easy to purify and can be polymerized directly by a catalytic ester interchange procedure to poly(methylenebenzoates), or can be hydrolyzed to p-and m-hydroxymethylbenzoic acids.
Anodic side-chain acetoxylation of methylbenzenes has been known (C. Magnussun et al., Chemica Scripta, 1, 57, (1971)). Benzene derivatives containing strongly electron-withdrawing substituents such as --COOCH.sub.3 are known in the prior art as not undergoing nuclear acetoxylation (Eberson, JACS, 89, 4672). Surprisingly, it has been found now that side-chain benzylic acetates can be formed with methyl toluate under the conditions of the instant invention despite the presence of a --COOCH.sub.3 group.
Ester interchange reactions are well known. For purposes of this invention, the term "ester interchange" is defined as a reaction between an ester and another compound, characterized by an exchange of alkoxy groups or of acyl groups, and resulting in the formation of a different ester. In the best-known types of ester interchange, the compound with which the ester reacts can be an alcohol (trans-alcoholysis), an acid (trans-acidolysis) or an ester (transester-ester exhange). The transester-ester interchange reaction has not been used as much as the other methods. In the absence of a catalyst, the formation of polyesters by the elimination of simple esters from fully esterified starting materials has been often considered as being too slow to be of practical value (I. Goodman et al., Polyesters, Pergaman Press, New York, 266 (1965)). Zinc acetate has been used as a catalyst to obtain poly (p-phenyleneterephthalate) from diphenyl terephthalate and p-diacetoxybenzene, magnesium as a catalyst with dimethyl terephthalate and 1,2-diacetoxyethane to obtain poly(ethyleneterephthalate) (Encyclopedia of Polymer Science and Technology, Interscience, New York, 11, 95 (1969)). Ethyl stearate and cetyl acetate reacted in an ester interchange reaction with sulfuric acid as a catalyst (V. V. Korshak, Polyesters, Pergamon, New York, 266 (1965)). In some cases, basic catalyst such as amines and potassium hydroxide are used, U.S. Pat. No. 3,039,994; Korshak, op. cit., 267.
Accordingly, it is an object of this invention to develop a method for side-chain acetoxylating methyl toluates. It is also an object of this invention to develop a method for purifying the side-chain acetoxylated methyl toluates, which are acetoxylated m- and p-hydroxymethylbenzoic acid compounds, to a purity required for polymerization use. It is also an object of this invention to teach a method for efficient polymerization of the said acetoxylated m- and p-hydroxymethylbenzoic acid compounds to produce high molecular weight homopolymers.